Sensors with cantilever and optical resonator are e.g. used in scanning force microscopy applications, in particular for atomic force microscopes or molecular force microscopes or other types of surface scanning force microscopes, but also in cantilever-based chemical, physical or biological sensors.
In this type of devices small changes of position of the cantilever, in particular due to a deflection, must be detected.
It is noteworthy that such a sensor can also be used to measure the deflection of the cantilever as a function of the position of the light spot on the cantilever. Therefore, different bending modes of the cantilever can be measured. This allows, for example, friction measurements (torsion of the cantilever) and the detection of various oscillation modes (longitudinal fundamental and higher orders as well as torsional).
D. Rugar et al. in Rev. Sci. Instrum. 59(11), 1988, pp. 2337ff and D. Rugar et al. in Appl. Phys. Lett. 55(25), 1989, pp. 2588ff and EP 398 085 describe a sensor head where the end of an optical fiber is arranged close to the cantilever. Light emitted from the fiber end is reflected from the cantilever and sent back into the fiber where it interferes with light reflected from the fiber-air interface. The interference pattern can be used to measure the position of the cantilever.
In another known embodiment, the sensor head comprises an optical resonator, a first mirror of which is arranged on the surface of the cantilever. Sensors with optical resonators are disclosed in U.S. Pat. No. 5,565,987 and U.S. Pat. No. 5,465,611. If the finesse of the resonator is sufficiently high, it can be used to detect even very fine movements of the cantilever.
However, it has been found that the stability of this type of resonators is poor and depends critically on the relative (Cartesian and angular) position of the cantilever and the sensor, which affects the reliability of the device. And, if a fiber is used, it has to be brought very close to the cantilever and strong movements of the cantilever can damage the fiber end.
In addition, electrostatic and other interactions between the fiber and the cantilever may occur that change the mechanical properties of the cantilever.